Their attacks require no human participation, and can run "neatly" with little chance of detection.

They also show elements of "insider levels of understanding".

Named 'Operation High Roller', the fraud has apparently targeted transfers from at least 60 banks.

Using automated transfer systems, the criminals have been able to steal money from high balance accounts under the noses of financial institutions, McAfee said.

Europe was the main target, the report claimed, but evidence of similar fraud in the United State and Colombia has also been found.

The attacks show criminals are moving more quickly, and are able to avoid detection for longer periods.

"The Operation High Roller attacks have impacted thousands of every class of financial institution: credit union, large global bank and regional bank, using smaller and less detectable automated transactions," the company said. "High net-worth individuals have also been targeted by the fraud ring."

"Unlike standard SpyEye and Zeus attacks that typically feature live (manual) interventions, we have discovered at least a dozen groups now using server-side components and heavy automation. The fraudsters' objective in these attacks is to siphon large amounts from high balance accounts."